It is a well known fact that in thermal power stations fired with pulverized coal, from an incombustible part of the pulverized coal blown into the boilers, a residue of combustion--slag and flyash--results in the form of a solid granular aggregate. Slag falling out directly from the furnace chamber has a grain size of 0 to 20.0 mm, while flyash is carried away together with flue gases and represents a residue of combustion of the grain size of 0.0-0.50 mm, as precipitated from flue gases. Slag and flyash contain mineral components which are characteristic of the coal used and versions may occur which arise in the oxidizing atmosphere because of thermal effects in the furnace chamber.
Most frequently occuring minerals in coal barren: Clay Minerals:
______________________________________ Kaolinite Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 Halloinite Al.sub.2 (OH).sub.n Si.sub.2 O.sub.5.2(H.sub.2 O) Montmorillonite Al.sub.2 (OH.sub.4)Si.sub.4 O.sub.10.H.sub.2 O Carbonate minerals: Limestone CaCO.sub.3 Siderite FeCO.sub.3 Magnesite MgCO.sub.3 Dolomite CaMg(CO.sub.3).sub.2 Ankerite FeMgMn(CaCO.sub.3).sub.2 Sulfide ores: Pyrite Fe.sub.2 S.sub.2 Melnikovite FeS.sub.2 (H.sub.2 O) Marcasite FeS.sub.2 Lead Sulfide PbS Copper quartz CuFe.sub.2 S.sub.2 Zinc sulfide ZnS Quartz SiO.sub.2 Salts: Common salt NaCl Sylvite KICl Gypsum CaSO.sub.4 2(H.sub.2 O) ______________________________________
In the furnace chamber of boilers clay minerals release absorbed water /H.sub.2 O/ in the form of steam, while iron sulfide ores release sulphur (S) in the form of a gas. Contained metals (AbFe, Cu, etc.), quartz (SiO.sub.2), salts (Na, K) melt in the furnace chamber and while being cooled, in the flow of flue gas, from tiny granular porous hollow melt spheres with gaseous inclusions, from quartz, vitreous spheres with gaseous inclusions are formed. From these polygonal needle-shaped flyash grains with sharp edges and confined by tiny plates are formed. Larger grains in a glowing state, falling out from the furnace chamber of the boiler into the cooling water below the boiler, wherein-as a consequence of quick cooling-porous slag grains with closed air inclusions are formed which have been shaped by steam. This latter phenomenon can be demonstrated by testing the density of slag grains. The density of slag grains formed from the same mineral depends on the size of the grain. For the sake of information we present the density of slag grains which fell out simultaneously from the furnace chamber of a power station. In respect to the order of magnitude these data characterize the density of slags coming from any power station fired with pulverized coal.
______________________________________ Sieve analysis Density of slag mm g/cm.sup.3 ______________________________________ &gt;10 1.014 10-6 1.053 6-3 1.150 3-1 1.629 1-0,5 1.647 0.05-0.25 1.824 0.25-0.1 2.012 0.1-0.063 2.120 below 0.063 2.176 ______________________________________
From these data it becomes obvious that in flyash, as well as in the slag, lower density of larger grains is caused mainly by the closed air-inclusions within the grains and by the porisity; accordingly, one of the characteristic physical feature of these grains lies in that they are not compact, are brittle and can be comminuted easily.
Carbonate minerals release carbon dioxide (CO.sub.2), iron oxides contained in the minerals also melt. At the same time Ca and Mg contents--in the bond CaO and MgO--appear in a solid aggregate, in fine grain structure both at the temperature of the furnace chamber and in a cooled state, mixed up with the grains of melts of metals, quartz and salts or adhering on their surface or are independently incorporated into the aggregate of slag and flyash, respectively.
Below we detail the characteristic mineral composition of slag and flyash:
______________________________________ SiO.sub.2 22-55% Fe.sub.2 O.sub.3 6-25% Al.sub.2 O 5-55% CAO 1-50% MgO 3-15% SO.sub.3 3-25% P.sub.2 O.sub.3 0-0.5% TiO.sub.3 0.2-2,5% K.sub.2 O 0.2-3% Na.sub.2 O 0.16% ______________________________________
Below we give the grain distribution of slag and flyash arising in a power station:
______________________________________ Grainsize Proportion of mass mm % ______________________________________ Grain distribution of flyash: &gt;0.5 0.02 0.5-0.25 0.95 0.25-0.1 15.36 0.1-0.063 14.89 0.063-0.030 24.18 below 0.03 44.60 Grain distribution of slag &gt;10 1.41 10-6 2.05 6-3 2.48 3-1 3.56 1-0.5 4.80 0.5-0.25 14.48 0.25-0.1 31.61 0.1-0.063 20.57 0.063-0.03 11.26 below 0.03 7.78 ______________________________________
Temperature of the slag grains falling out from the furnace chamber will be determined by the temperature of the water in the basin below the furnace chamber for cooling the slag. In practice temperatures used lie in the range between 30.degree. and 60.degree. C.
The temperature of the flyash grains precipitated in bulk from the flue gases is equal approximately to the temperature of the flue gas. That means that in practice flyash grains with a temperature of 80.degree.-140.degree. C. arrive into the collecting bins of the precipitators.
As a consequence of mineral composition, temperature, shape of grains and structural layout of grains and grain structure, composition of the aggregate of collected slag and flyash, slags and flyashes have different physical and chemical properties.
Some of these properties have been utilized in cement production, the building industry, road construction etc. Knowledge of physical and chemical characteristics is imperative for delivering slag and flyash in mass mixed with water in pipelines and for deposition of them in aggregates of large area and extent.
In accordance with earlier practice in thermal power stations slag formed in the furnace chamber is allowed to fall into the water bath below the boilers, where it is cooled and some larger, possibly fused-together pieces are comminuted in breakers. Flyash with coarse grains precipitated mechanically from flue gases was flushed away in a water flow. Flyash with finer grains precipitated by means of electrofilters or filterbags from flue gases is generally collected in a dry state in air flow for using it--in consideration of their resultant properties--in cement plants as fillers or in plants producing cellular concrete as an additive etc.
According to general practice fine flyash in a dry state not having utilized yet could be mixed with slag, coarse flyash and water and delivered in pipelines to final depository places. As a consequence of admixing of slag flyash and water, some physical characteristics and the chemical composition of the individual components will change. Water cools the grains and elutriates mineral salts contained therein. To the extent there is elution physical and chemical properties of the slag and flyash grains, and the aggregate thereof will change. It goes without saying that mineral salts having been eluted from slags and flyash change compositions of the forwarding water and certain characteristics thereof.
Processes are known, in which slag and flyash has been collected within the thermal power plant and mixed with water; thereafter the mixture thus obtained is delivered through a pipeline to the deposition site, wherein the mixture is spread by means of surface stream and/or grains of slag are precipitated from the mixture. These processes are known as hydraulic collecting-transporting and depositing processes.
An earlier and frequently used process has in, in considerable water consumption and slag and flyash are collected, delivered and deposited in a considerably diluted state; in course of this process within the boiler slag and flyash are collected and in consideration of technical aspects of collecting equipments, collection is realized with a considerable water flow in the proportion 1:2-1:20 weight-% (solid matter:liquid). The highly diluted mixture thus obtained is delivered through a pressurized pipeline to the place of deposition. Here solid and liquid phases are separated by precipitation with a large water surface. Slag and flyash--now in a solid state--are deposited here, while the liquid phase is led to a collecting basin from the place of deposition and, in certain cases, may leak into the subsoil.
Hydraulic processes for collecting, delivering and depositing slag and flyash, in course of which a water quanity is equals in weight to the solid material aggregate, or it is even less, so that the content of solid matter is large, are considered as relatively new processes. In course of these processes the water quantity added to slags and flyashes is not more, than is needed for its hydraulic binding ability, i.e. hardening of the deposited material.
In this case a mixing process has been used, as a result of which the mixture of slag-flyash-water has characteristics of heavy liquids. From the point of view of delivering through a pipeline it can be stated that physical characteristics of slag and flyash grains being in a bulk state, namely size and density, enable delivery in a pipeline, if mixed with water.
It is a well-known fact that when allowing a sludge to flow in a pipeline, which is diluted with a considerable water quantity, i.e. in a proportion of 1:2 and 1:10 weight-% (solid matter-liquid), the sludge has flow properties, which are characteristic for the flow of heterogenous mixtures.
If the mixture does not flow with the proper velocity, as a consequence of the weight of larger grains the mixture will be separated to its original components. That means that liquid phase and solid matter are separated and solid grains precipitate in the pipeline. Hence this type of mixture can be allowed to flow in a pipeline with a flow velocity only, in the so-called turbulent range. The extent of turbulence, the so-called "critical velocity" can be characterized by the magnitude which keeps even the largest grains in the sludge in motion. Accordingly, the so-called "critical" velocity represents the lowest limit of turbulent flow. In the case of delivery of slag and flyash this value lies in the range between 1.5-1.6 m/sec. Below this limit delivery of slag and flyash, respectively, stops, in that in the pipeline larger solid grains no longer flow. That means that flow velocity of the mixture is to be kept above the critical velocity. If the content of solid matter becomes less, the missing quantity is to be replaced with water. For this reason there is no possibility to change the quantity of a mixture of constant density within a given pipeline within wide limits. As already mentioned, the lowest limit of the mixture to be delivered in the pipeline will be determined by the "critical velocity" for the diameter of the pipeline and grain composition of slag and flyash.
It is a well known fact that in course of the mixing process for preparing sludges with water quantities which are equal in weight to the solid matter aggregate or even less, i.e. 1:1-3:1 weight % solid matter liquid, special physical characteristics of fine flyash grains are utilized, as only with an aggregate with such a grain distribution and grain composition--0.0-0.50 mm--is it possible to obtain uniform distribution with a small quantity of water in the aggregate, by performing dynamic agitation to obtain a water layer of so-called molecular thickness on the surface of grains. As a consequence of dynamic mixing the mixture of flyash and water becomes liquefied--with the characteristics of heavy liquids--so it can be pumped and allowed to flow in a pipeline.
It is a well known physical phenomenon, that in the course of delivering liquid substances in a pipeline, temperature of the liquid medium influences the so called "loss of pipe friction" and accordingly economic parameters of the pipeline, as transporting means. Increased temperature reduces viscosity of liquid materials. Considerable change in viscosity appears mainly in liquids with higher viscosity (e.g. crude oil, residual oil, etc.).
With means delivering slag and flyash using a considerable water quantity, and slag and flyash of high temperature, the temperature of the mixture prepared with a large water quantity does not increase to such an extent, that one had to reckon with the effect of change in temperature, since the viscosity approaches that of clean water.
It has been proposed to provide thinner pipelines 100-200 mm--with heat insulation or to lay them into the soil to protect the line against freezing up, as it may happen that a mixture of 10.degree. to 30.degree. C. may freeze up on cold days and high velocity of wind, mainly in longdistance pipelines (10-20 km).
Ecological processes are also known. We, in this category, describe technical solutions seeking to eliminate dust contamination of the air from the surface of slag-flyash deposits of large volume and area and the elimination of chemical contamination of ground water and subsoil by waters leaking through the aggregate.
In order to be able to eliminate dust contamination, the surface of the deposition has to be covered with water or one must establish surfaces free of dust. To prevent contamination of subsoil and waters in the subsoil--having mostly drinking quality--water storing spaces must be isolated in a waterproof way so that the protection should yield proper safety even after stopping operation of the deposit.
As for deposit, one of the most characteristic features of slags and flyashes appears in the so called "hydraulic binding ability" resulting from the carbonate containing mineral components and size of the grains, furtheron, resulting from the grain distribution of slag and flyash aggregates and structure of the grains, with a majority of deposits. The joint-factor characterizes density and "permeability to water" or watertightness. These are in a close connection with one another, determining interconnection between deposits of large area and the atmosphere, environmental soil layers, as well as flow of ground water.
The hydraulic binding ability of the slag and flyash--in the case of a low content of CaO and MgO--does not develop when admixed with a considerable quantity of water in the traditional way. As a consequence grains of slag and flyash do not stick to each other in the deposit, so that air streams carry fine grains away from the dried surface of the deposit. That means that the surface of the deposit emits clouds of dust. At the same time, in the course of precipitation from water, grains of slag and flyash are separated according to grain size in compliance with topographic potentialities of the storing space. Accordingly, in course of precipitation deposits will be built-up of aggregates or layers of different grain sizes. Hence the factor of permeability to water, i.e. 5=10.sup.-2 to 10.sup.-3 cm/sec--will be different too. The joint coefficient, characterizing density of deposits, will be e=0.9 to 4.0.
With systems operated with the mixture of slag, flyash and water, with considerable dilution, a part of the salts released from slag and flyash gets into the forwarding water, while dissolved salts contaminate ground water flowing in the subsoil with undesired mineral components and trace elements. Deposits are found in mine pits, in the path of flow of surrounding ground water. They are interconnected with soil layers conducting ground water. Accordingly, contamination from the deposits can be reckoned with permanently.
According to experience, deposits consisting of the mixture of slag-flyash-water with a significant content of solid matter will never correspond to an aggregate of grains with a loose constitution. However, in dependence upon CaO and MgO content, compressive strength of 1 to 5.0 kg/cm.sup.2 can be measured. An aggregate of this kind has a coefficient of permeability to water which is less by several orders of magnitude, so K=10.sup.-5 to 10.sup.-6 cm/sec. There is no precipitating lake with a large water surface in the deposit space. Accordingly water containing harmful mineral salts does not leak into the subsoil and clouds of dust do not form on the surface of the deposit. As a consequence, deposit can be harmonized with environmental requirements. Although in this case the surface of the deposit is not covered by a water surface, but by virtue of adhesion between the grains the deposit will have a hard surface, dust formation stops. Within the deposits grains are not separated. Accordingly the aggregate of slag and flyash shows a homogeneous structure from the point of view of permeability, density of the deposit can be characterized by a joint-coefficient of e=0.45 to 0.85. The aforementioned coefficient relating to permeability to water--i.e. K=10.sup.-5 to 10.sup.-6 cm/sec--does not mean an unambigous water-tightness as a part of rainwater and water from thawing of snow will leak through the deposit into the subsoil or ground water and which can be polluted by dissolved mineral salts.
Processes for producing homogeneous sludges are also known. As an example let us mention the process as described in the Polish Patent 185,413 in course of which dry flyash and water are charged into a mixer where it is mixed. By the alternating operation of at least two pieces of equipment a mixture is produced in a proportion of 1:1 to 3:1 (flyash-water). This process is completed by the process as described in another Polish Patent Specification 185 795, in the course of which into the mixture of flyash and water--as obtained from the aforementioned mixer--slag is admixed, in such a manner the mixture thus obtained is delivered to the deposit area through a pipeline.
A further process is known from the Polish Patent Specification 245 199, in course of which a mixture consisting of slag-flyash-water with a high content of solid matter is produced, and so that mixing flyash with water is effected continuously, while in the final phase of the process slag formed in the furnace chamber is admixed too. The solution is completed by the Polish Patent Specification 246,465, which uses a mixer for realizing the aforementioned process.
Hungarian Patent Applications 7293/83, 7928/84 and 15492/87 are considered as developments of the above mentioned Polish Patents since they describe continuously operated equipment and highly concentrated mixture of slag-flyash-water are produced by the previous mixing of slag with water.
The Polish Patent Specification 181 295 relates to the deposition of slag and flyash coming from thermal power stations. This process can be characterized in that the mixture of flyash and water, containing solid matter in a high proportion 1:1 -2.5:1 is led to the place of deposition, wherein coefficient of permeability to water max, K=10.sup.-7 cm/sec and compressive strength equals a min. is 3.0 kg/cm.sup.2.
If environmental conditions require a higher strength, known binding materials, so e.g. hydraulic lime, slacked lime and other chemical agents could be added in a quantity of 6 weight %. These ingredients increase or replace the effect of binding materials, like CaO and MgO.
Hungarian Patent Application 2343/88 yields the possibility of depositing a mixture consisting of slag and flyash containing a considerable quantity of solid matter, the aim is to utilize flow properties of viscous liquids.
According to the Polish Patent Specifications an indispensable requirement is in that in course of delivering the flyash-water mixture with a high content of solid matter and with the subsequently admixed slag, delivery in a pipeline requires a flow with the velocity of 1.1.about.1.2. Below this velocity larger slag grains precipitate and will be separated from the flyash-water mixture. If there is no possibility to establish the minimal velocity of flow, discontinuous operation gives the right solution. That means that one has to provide for buffer storage, in which flyash and slag are stored in a dry state as long as the quantity is staying at disposal which can be forwarded in a given pipeline with the required velocity.
It can be stated that for both mixing processes according to the Polish Patent Specification the use of so called industrial water was proposed for producing the mixture of flyash and water. This water comes from the water supply system of the thermal power station. Since the temperature of the water corresponds mostly to ambient temperature, the water cools the flyash-aggregate of high temperature to such an extent, that the pipeline--delivering to a longer distance--must be protected against frost because the heat contained in slag and flyash cannot be utilized.
It can be stated that none of the Hungarian Patents--relating to processes and equipments--contains elements by the aid of which unrestricted change of mixture flow within a given pipeline can be realized unambigously, enabling production of a mixture with higher temperature and to reduce permeability to water on the single deposits to such an extent that the deposit could be qualified as watertight.
It can be stated that binding materials containing CaO and MgO require carbon dioxide (CO.sub.2) contained in air, to assure hydraulic i.e. carbonate, binding. However, as shown by experience--in deposits of considerable thickness binding cannot be realized in the absence of air. Accordingly, even if additives are added in accordance with the Polish Patent Specification 221 769, neither higher strength, nor complete watertightness can be achieved in deposits with thicker layers. Hardness of a carbonation origin will be formed but on the surface of the deposit, the importance manifests in the dust-free surface of the deposit.